Orotidine-5'-phosphate Decarboxylase. We have determined the binding to orotidine-5'-phosphate decarboxylase of azauridine-5'-phosphate relative to that of the corresponding barbiturate. Although the inhibitor constant of the former has been reported, the value is not reliable; it will now be determined accurately by a relay system, linking it to uridine-5'-phosphate. The rates of dissociation of the inhibitor are known, so that the "on" rate will be calculated. We propose to synthesize analogs of the barbiturate inhibitor to test the hypothesis that it is a transition state analog and will prepare other analogs that may prove useful for photoaffinity labeling of the enzyme. We have previously prepared an affinity column for the purification of orotidine-5'-phosphate decarboxylase; we now wish to try this column and the barbiturate inhibitor with other enzymes of pyrimidine metabolism. Photoaffinity Labeling. New photoaffinity labeling reagents related to dimethylaminonaphthalene have been prepared; these will be tested in model systems. Chiral Methyl. Mr. Steven Benner and Mr. David Rozzell plan to prepare chiral acetic acid (CHDT-CO2H) and chiral methanol (CHDTOH) by a novel scheme that makes use of the stereospecificity of acetoacetate decarboxylase.